Polyvinyl chloride (PVC), by virtue of a particularly desirable combination of properties, including non-flammability, high chemical resistance, high strength and good weather resistance is one of the largest volume polymers, and it is probably the most versatile in the scope of its utility. PVC is essentially a glassy or amorphous polymer with a high glass transition temperature (T.sub.g) of about 80.degree. C., and thus alone it is rigid and brittle at use temperatures up to about 80.degree. C., and particularly at the usual ambient temperatures. However, PVC can be made flexible and soft by adding a plasticizer typical of which is dioctyl phthalate (DOP). DOP, as with most plasticizers, and especially carboxylic acid esters, is a liquid at room temperature and acts as a weak solvent for PVC. The thus plasticized and flexible PVC, to be called SPVC, is used for vinyl seat covers, artificial leather, clothing items, etc. But for certain important purposes, PVC must be used without plasticizers. For instance, unplasticized rigid PVC, to be called RPVC, is used for vinyl siding, drainage and sewer pipe, water and electrical conduits and fittings, outdoor furniture, etc.
PVC is indeed an extremely versatile polymer and serves as an major industrial and consumer material for modern man. However, as is well known PVC presents serious difficulties in processing during fabrication, shaping, etc., and RPVC is peculiarly difficult to process. For manipulation during fabrication, RPVC because of a very high melt viscosity must be heated to at least about 210.degree. C. to achieve complete fusion as is needed to impart optimum properties to the final product especially when produced by extrusion or injection molding. Unfortunately, PVC is thermally very unstable at about 210.degree. C. and even considerably lower temperatures. Sarvetnick in "Polyvinyl Chloride", Van Nostrand Co., 1969, Page 90, states that the thermal degradation of PVC begins at about 93.3.degree. C. and increases sharply with increasing temperature. At practical processing temperature, it quickly undergoes degradation, giving off very toxic and corrosive hydrochloric acid (HCl) fumes with serious loss in mechanical properties. Excessive scrap due to degradation, low production rates and poor product properties are notorious problems encountered in PVC processing.
As one way of improving the thermal stability as well as other properties of PVC, copolymers containing a major fraction of vinyl chloride units and a minor fraction of other monomeric units, such as vinyl acetate, serving to soften and flexibilize the ultimate polymer and reduce its glass transition or brittle temperature, have been developed and are widely used. Also, many processing aids, e.g. stabilizers, lubricants, impact modifiers, and the like also have been developed for PVC in order to facilitate processing at lower temperatures or to reduce the tendency of the polymer to undergo heat decomposition or impart more favorable properties. These aids must often be used in large amounts, up to 30-40% by wt of the polymer in the case of impact modifiers, and while these levels are tolerable or even desirable for SPVC, they are not acceptable for RPVC because they usually result in losses in essential properties.
The objective of this invention is to provide an improved processing aid and methods of utilizing such aids for vinyl chloride polymers which enable the same to be processed at lower temperatures without major adverse side effects. Selective low molecular weight crystalline chemical compounds have been found to behave as solid solvents to PVC when they are mixed therewith, qualifying as effective processing aids for PVC. A solid solvent is different in function from any of a plasticizer, lubricant or impact modifier. Thus, a solid solvent in homogenous admixture with PVC acts as solvent for the PVC, with the capacity for greatly reducing the melt viscosity of the PVC, only when the admixture is at high temperatures where the mixture is fused during processing. But when the mixture is cooled to room temperatures below the melting point (T.sub.m) of the solid solvent and the T.sub.g of the polymer, the former precipitates out of the polymer as microdispersed solid micro-particles. Therefore, if the amount of solid solvent is controlled within proper limits, its presence does not adversely reduce the strength, rigidity or useful temperature range of solid PVC, especially RPVC. A solid solvent must possess thermally reversible compatibility or solubility with PVC. A plasticizer, on the other hand dissolves permanently into PVC, while a lubricant is a mechanically entrained friction-reducing additive that is basically incompatible with the polymer and does not act as solvent to it. Both therefore tend to impair desirable mechanical properties of the polymer in contrast to a solid solvent.
In a paper "A Solid Solvent as Processing Aid for Polystyrene," J. Applied Polymer Research, Vol. 37, 1339-1349 (1989), Chung et al. describe their investigation for an effective solid solvent for polystyrene (PS) which resulted in the identification of benzenesulfonamide (BSA) as an apparently ideal material for that purpose. BSA was found when incorporated at the 5% by wt level to reduce the melt viscosity of a sample of a typical commercial PS with normal molecular weight (Number average=87,000; Weight average=230,000; T.sub.g =379.degree. K.) by about 60%, i.e. from about 5,000 Pa-s for PS alone to about 2,000 Pa-s when BSA was present when tested at 490.degree. K. at a low shear rate of about 4/S. A heat capacity measurement of powdered BSA in association with a finely divided low molecular weight PS fraction (MW=10,000; T.sub.g =367.degree. K.) was carried out in a differential scanning calorimeter (DSC) wherein transitions at either of the melting or glass transition points appear as deflections, e.g. peaks or shoulder-like increases in slope of the heat curve output of the DSC. Upon reheating of the DSC after once undergoing homogeneous melt mixing with the PS fraction and subsequent quenching, i.e. rapid cooling, the BSA caused roughly a 10.degree. C. downshifting or drop in the T.sub.g of the PS fraction while retaining intact to a major degree the area of its own melting peak at its distinctive crystalline melting point as was developed during the initial melt mixing. The latter behavior signifies the presence of BSA in the quenched mixture as a distinct crystalline dispersed phase. In contrast for comparison, the same amount of mineral oil, which acts as a plasticizer for PS, while imparting during DSC analysis an even greater reduction in the T.sub.g of the low molecular wt. PS, i.e. of about 20.degree. C., was able to reduce the viscosity of the high molecular wt. PS sample under the same viscosity test conditions only about half as much, i.e., from about 5,000 Pa-s to about 3,500 Pa-s. Mineral oil, being liquid over the test temperature range, of course showed no melting peak by DSC analysis, either during initial melt mixing or subsequent re-heating.
Also as a comparison, acetanilide (AA), with a crystalline T.sub.m of 115.degree. C. only a little higher than the T.sub.g of PS, which had earlier being thought as a promising solid solvent candidate for PS, as reported by Chung in J. Applied Polymer Sciences, 31, 2739 (1986), was found inferior to BSA but superior to mineral oil in reducing melt viscosity of the same high molecular weight sample of PS to about 3,000 Pa-s. Upon DSC analysis with the low molecular PS sample, AA while achieving the greatest downshift of the three additives in the T.sub.g of that sample of at least 30.degree. C., suffered during re-heating a major but not complete loss in the area of its melting peak, denoting substantial lasting solubility in the polymer after quenching of the latter to its solid state. AA by DSC testing alone was found to exhibit a wide hysteresis effect in its heat flow curves upon melting and subsequent rapid cooling alone in the absence of annealing, which effect was much greater than that of BSA when similarly treated. This hysteresis effect depressed the apparent recrystallization temperature (T.sub.c) of AA below the T.sub.g of the PS sample, suggesting that recrystallizing ability of AA within the solidified PS was being hindered by its low apparent T.sub.c.
Some 30 additional low molecular weight crystalline compounds, all with T.sub.m significantly higher than the T.sub.g of PS, that were evaluated by DSC during this study, and of these, only four were found to have promise as solid solvent for PS, including two carboxylic acid amide plus sebacic acid and mannitol. As noted, all DSC tests were conducted using the low MW fraction of PS having a T.sub.g significantly lower (by 12.degree. C.) than that of the commercial PS sample. This difference in T.sub.g due to the difference in MW of PS would be expected from the phase diagram of that polymer reported in "Plasticization and Plasticizer Processes," a symposium report No. 48 of The Advances in Chemistry Series, American Chemical Society, Washington, D.C., 1965, Page 39. In as much as low MW grades of PS are themselves known to function as internal plasticizers for normal high MW general purpose grades of PS, acting to improve the flow rate while increasing the brittleness thereof (Cf. "Polymer Technology" by Miles and Briston, Chemical Publishing Co., Inc., New York, 1965, Page 188), the effectiveness of any of these other compounds in actually reducing the melt viscosity of PS in the normal MW range, is speculative at best. In particular, extrapolation as to such effectiveness between chemically very different materials within the usual practical MW range based on DSC data is obviously out of the question.
The solid solvent concept was applied by Chung in U.S. Pat. No. 4,843,117 to vinylidene chloride-containing polymers. e.g., copolymers of vinylidene chloride and a minor amount of vinyl chloride or methyl acetate. Dimethyl sulfone at a concentration of 5% was found to be an effective solid solvent for such polymers, reducing melt viscosity e.g. by almost 40%, i.e, from about 10,000 Poise to about 6,000 Poise, at a low shear rate of 100/sec and 175.degree. C.
While PS can under special polymerizing conditions with Ziegler catalysts be produced as an isotactic crystalline polymer with a high crystalline T.sub.m of about 230.degree. C., (Cf, Miles and Briston, supra, Page 187), PS as ordinarily used is an amorphous polymer, like PVC, with a high T.sub.g of about 100.degree. C. Its intrinsic brittleness can be readily modified for improved toughness. But unlike PVC, it is quite fluid, e.g., suitable for good injection molding, at 217.degree. C. (490.degree. K.) as used by Chung et al. PS is free of any tendency to suffer decomposition during processing.
Polyvinylidene chloride (PVDC) being highly crystalline with a true T.sub.m of 190.degree. C. and a quite low T.sub.g of -19.degree. C., while lacking the brittleness of PVC at ordinary use temperatures, is even more difficult to process in its pure form and more susceptible to heat degradation than PVC, and so in that form is of no commercial importance (Cf, "Manufacture of Plastics", by Smith, Reinhold publishing, New York, 1964, pp. 337-339). For commercial purposes, PVDC is always copolymerized with a modifying co-monomer, such as vinyl chloride, vinyl acetate, acrylonitrile, etc., and the resultant co-polymers have a reduced melt temperature. For example, Park in "Plastics Film Technology", Van Nostrand-Reinhold Co., 1969, Pages 35 & 36. describes a bubble process for forming a film of a PVDC-PVC copolymer (85/15) in which the copolymer is satisfactorily extruded at 170.degree. C. Such film, which is sold commercially in large quantities under the trademark "SARAN", has excellent optical clarity and high tensile strength. Also, in the Example of the above-identified U.S. Pat. No. 4,843,117 to Chung, the PVDC copolymer had a melt temperature of about 175.degree. C. At such temperatures, the decomposition problem of PVDC copolymers is serious.
There is a need in practice to develop processing aids for RPVC to address the unavoidable characteristics of use temperature rigidity and processing temperature decomposition of that polymer. The identification of workable solid solvents for RPVC is a more pressing problem than for either of PS or of PVDC copolymers because of the large volume of PVC in use.
I have now discovered that low molecular weight crystalline carboxylic acids and their simple derivatives which have a crystalline melting point at least equal to the T.sub.g of about 80.degree. C. for PVC, and preferably higher, but not higher than about 180.degree. C. are useful solid solvents for rigid PVC polymers consisting essentially of vinyl chloride. This is surprising since, in the first place, amorphous polymers, as is PVC, have a strong tendency towards permanent solvation with compounds which exert a plasticizing action thereon. As stated in the ACS symposium report "Advances in Chemistry Series", No. 48, supra, at Page 3: "With an amorphous polymer, any plasticizer is a solvent plasticizer--i.e., under suitable conditions the polymers would eventually dissolve in the plasticizer." This holds true for common closely related carboxylic acid esters and diesters which are liquids at room temperature and are among the most widely used solvent plasticizers for various polymers, notably PVC. In the second place, the compounds found advantageous for RPVC are chemically different from those found effective for PS and PVDC co-polymers, respectively, namely BSA and dimethyl sulfone, as described above, and the latter proved unsuitable as solid solvents for RPVC.